1. Field of the Invention
The present invention relates to a charging device capable of obviating abnormal sound having several frequency peaks ascribable to charging without any irregular charging, and an image forming apparatus including the same.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming apparatus to optically scan the charged surface of a photoconductive drum or image carrier for thereby forming a latent image, deposit toner on the latent image to thereby form a toner image, and transfer the toner image to a sheet or recording medium. To charge the surface of the photoconductive drum, the electrophotographic image forming apparatus has traditionally used a corotron, scorotron or similar wire charging system. However, the wire charging system produces some ozone at the time of charging and is therefore undesirable from an environmental standpoint.
In light of the above, a charging system of the type holding a charge roller or similar charging member, which does not produce ozone, in contact with the photoconductive drum has been proposed. This type of charging system, however, has a problem that when a DC voltage is applied between the charging member and the drum alone, irregular charging occurs. To solve this problem, an AC voltage is usually superposed on the DC voltage to allow the charging member to uniformly charge the surface of the drum.
However, the AC-biased DC voltage mentioned above brings about another problem that an electric field is formed between the charging member applied with the AC voltage and the drum not applied with the AC voltage. The electric field thus formed causes the drum and charging member to repeatedly attract each other, resulting in oscillation between the drum and the charging member. Consequently, at the time of charging, the drum and charging member knock against each other due to the oscillation, producing abnormal sound. The abnormal sound occurs at the frequency of the AC voltage applied and frequencies which are the multiples of the above frequency, as known in the art.
Various technologies have heretofore been proposed to obviate abnormal sound ascribable to the superposition of the AC voltage on the DC voltage, as will be briefly described hereinafter.
Japanese Patent Laid-Open Publication No. 2000-206762, for example, proposes to reduce the oscillation of the surface of the charge roller relative to that of the drum to 150 μm or below and to provide the charge roller with a unique configuration. Japanese Patent Laid-Open Publication No. 2000-330360 teaches that at least NOR (polynorbornene rubber) is contained in the conductive rubber layer of a charge roller.
Japanese Patent Laid-Open Publication No. 5-3505 proposes to establish the following relations between the specific gravity ρ of a photoconductive drum and the frequency f of oscillation voltage applied between the drum and a charge roller:ρ≧1.4×10−1·f (f≦350 Hz)ρ≧4.0×10−4·f+0.35 (350 Hz<f≦1,500 Hz)ρ≧0.95 (f>1,500 Hz)
Japanese Patent Laid-Open Publication No. 5-142921 teaches that a cylinder formed of urethane rubber whose thermal conductivity is 10 W/m·K or below is disposed inside a photoconductive drum (subject of discharge) in contact with the inner periphery of the drum so as to increase the weight and rigidity of the drum. Likewise, Japanese Patent Laid-Open Publication No. 5-142922 proposes to insert, e.g., a rigid or an elastic body in a photoconductive drum and affix the former to the latter to thereby increase the weight and rigidity of the drum. Further, Japanese Patent Laid-Open Publication Nos. 5-188838, 5-188839 and 5-188840 propose respective schemes for the purpose described above.
However, none of the conventional technologies described above can fully obviate abnormal sound, or charging sound, ascribable to the superposition of the AC voltage on the DC voltage.
There has also been proposed to reduce the frequency of the AC voltage to be applied to a charging member to 10 Hz to 500 Hz in order to reduce abnormal sound. This, however, gives rise to another problem that the charging of the image carrier becomes irregular, resulting in an irregular image. Further, when it comes to a digital PPC (Plain Paper Copier) or a laser printer using a laser beam or an LED (Light Emitting Diode Array), the frequency of the AC voltage as low as 100 Hz to 500 Hz, which is close to a multiple of pixel density, causes moiré to appear in an image.
Moiré cannot be obviated unless the frequency or charging frequency of the AC voltage to be superposed is higher than interference frequency, which is determined by pixel density and process speed. For example, when pixel density and process speed are 600 dpi (dots per inch) and 100 mm/sec, respectively, the charging frequency should be about 1,000 Hz. A future electrophotographic apparatus having a high pixel density, high process speed configuration is expected to need charging frequency of at least about 1,500 Hz. In this respect, the frequency of the AC voltage to be applied to the charging member cannot be lowered below a certain limit.
Moreover, assume that the AC voltage to be superposed on the DC voltage is set such that the peak-to-peak voltage is not higher than two times or more of a charge start voltage. Then, although abnormal noise ascribable to charging can be reduced, sufficient charge cannot be applied to the photoconductive drum with the result that irregular charging is apt to appear on the drum surface in the form of spots. Irregular charging renders an image irregular, as stated earlier.